If you want both values populated - you first need to actually calculate what they should be based on the other entry. So you would make % and Amount calculate fields with If statements that determine what the value(s) should be. For example on the % side that says if the % is the field that is entered, then your calculated field would just copy that value same value. If the manual amount field is entered instead then you would do the amount / rate to return the % based on the entered amount.
With those calculated fields you can then copy them into the input fields using a form rule or pipeline.
